The present invention relates to wireless telecommunication systems, and more particularly to a system and method for maintaining synchronization and frequency stability in a wireless communication device of a telecommunication system PBX where the clock for the PBX does not consistently provide an input to the wireless device which has adequate frequency stability.
With reference to FIG. 1, telecommunication systems may include a private exchange (PBX) 8 for providing telephone service to a local network of telephones, such as in an office. PBX 8 may be connected to its network of telephones 12 through cables, and more recently to telephones 14 through wireless connections. The expansion of the traditional PBX into wireless communication has, however, brought out the need for improvements to portions of the PBX 8. One area for improvement is in the wireless device (WPBX) 16 of PBX 8 which provides the wireless connections and operates with frequencies which must be more stable than was needed for cable communication. That is, the stability of a system clock input CLK 34 from a system clock in PBX 8 is satisfactory for cable communication, but not for wireless communication.
The source of wireless device clock input CLK 34 is generally phase locked to PBX 8 which is synchronized to a network clock 22 (via a digital connection, T1/E1) or alternately wireless device is synchronized to the PBX 8 that is operating with a standalone reference clock.
By way of example, a wireless PBX communication may require a frequency stability of better than plus or minus 4.6 parts per million. However, wireless device clock input CLK 34 may meet this frequency stability requirement part of the time. During other periods of time the frequency of wireless device clock input CLK 34 is not stable enough for use in wireless device 16. With reference to FIG. 2, one solution would be to upgrade the source of network clock input 22 to meet the frequency stability requirement of wireless device 16. This is not always practical in all PBX System configurations and installations. Some configurations may not have digital connections to the network and therefore will operate as a stand alone system. A second solution would be to provide a PBX PLL, 36 that meets or exceeds the wireless device clock requirements. This is not only a costly implementation but also not desirable because this PLL normally is required to track a network clock of lower precision.
Accordingly, it is an object of the present invention to provide a novel system and method for providing a wireless device in a telecommunication system with a stable frequency and synchronization which obviates the problems of the prior art.
It is another object of the present invention to provide a novel system and method for maintaining frequency stability in a wireless device of a telecommunication system which uses a system clock CLK 34 for normal operation thereof and in which the wireless device is operated with reference to a precision oscillator, 28 that has a frequency stability which meets or exceeds the frequency stability requirement of the wireless device.
It is yet another object of the present invention to provide a novel system and method for maintaining frequency stability in a wireless device of a telecommunication system where the PBX PLL 32 maintains the frequency synchronization to a network clock input 22, and in which the frequency stability of the system clock 32 with reference to the precision oscillator is frequency evaluated 30 to insure stability within the limits required by the wireless device 16.
It is still another object of the present invention to provide a novel system and method for maintaining frequency stability in a wireless device of a telecommunication system in which the frequency stability of the system clock 32 is longer within the limits of the wireless clock 34. The stability evaluator 30 in conjunction with the precision oscillator 28 maintains the frequency stability requirement of the wireless device 16. The frequency synchronization is not maintained to the system clock 32 during this time. It is a further object of the present invention to provide a novel system and method for maintaining frequency stability in a wireless device of a telecommunication system in which the difference in clock synchronization between the PBX PLL clock 32 and the wireless device clock 34 are buffered in an elastic store to minimize frame slips between the wireless system and the PBX.
It is still a further object of the present invention to provide a novel system and method for maintaining frequency stability in a wireless device of a telecommunication system in which the frequency synchronizer 33 in conjunction with the stability evaluator 30 are used to adjust the frequency of the wireless clock 34 to increase or decrease the period during which the frequency stability of the wireless clock 34 continues to meet the frequency stability requirement of the wireless device.
It is yet a further object of the present invention to provide a novel system and method for maintaining frequency stability in a wireless device of a telecommunication system which the frequency stability evaluator 30 tracks the PBX PLL 32 or network clock 22 while the frequency stability is within the limits of the wireless device requirements. The frequency stability evaluator maintains the frequency stability of the wireless device clock 34 at the limit as long as the PBX PLL 32 or network clock 22 are outside the requirement of the wireless device and will track the PBX PLL or network clock when these clocks are again within the stability requirements on the wireless device.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.